One of the problems which is encountered with concrete mixers, particularly of the so-called "turbine" type, is the difficulty of maintaining the lugs or blades clean, since concrete tends to pack between the lugs and the supporting surfaces. This difficulty is most noticeable when the concrete is of the "dry mix" type, and commonly a mixer requires cleaning twice a day. Until such time as it is cleaned, the mixer gets less and less efficient. The cleaning however is a time consuming unpleasant and expensive operation.
Another problem which is encountered is that there is a high wear rate on the ends of metal lugs or blades, and relatively low through-put in many cases.
Another problem which is encountered is that turbine mixers absorb very large amounts of power, and for example a mixer of sufficient size to have a through-put of 800 kg per minute can absorb as much as 60 h.p.
Even more important than the abovementioned difficulties however is the difficulty that many mixers have in mixing some dry mixes without segregation or without aeration. When the concrete is poured, extruded or pressed, quite often the air is not released sufficiently to ensure that the resultant product has maximum density, and there is a consequential loss of mechanical strength. This difficulty is a major difficulty with concrete having only a small water content, and it is a difficulty which has not been completely solved with previous machines.
One object of this invention is to provide a method of mixing the ingredients of Portland cement based concrete, and to provide a mixer which is capable of mixing concrete by that method, whereby the above-mentioned problems can all be reduced to some extent at least.